| `source_span` is a library for tracking locations in source code. It's designed |
| to provide a standard representation for source code locations and spans so that |
| disparate packages can easily pass them among one another, and to make it easy |
| to generate human-friendly messages associated with a given piece of code. |
| The most commonly-used class is the package's namesake, `SourceSpan`. It |
| represents a span of characters in some source file, and is often attached to an |
| object that has been parsed to indicate where it was parsed from. It provides |
| access to the text of the span via `SourceSpan.text` and can be used to produce |
| human-friendly messages using `SourceSpan.message()`. |
| When parsing code from a file, `SourceFile` is useful. Not only does it provide |
| an efficient means of computing line and column numbers, `SourceFile.span()` |
| returns special `FileSpan`s that are able to provide more context for their |
